Welding of sheet metal coated with layers

ABSTRACT

A method for the connection of sheet metal panels coated on at least one surface with an insulating layer by the electrical fusion of a metal, wherein the improvement comprises the steps of supplying an electric current for the fusion of metal indirectly along the sheet metal of at least one panel to the zone at which said panels are to be connected to each other, and keeping the temperature of an outer insulating layer of at least one of said panels below the melting point of said layer.

United States Patent Inventor Otto Alfred Becker 59 Robert Kock Strasse D-6600, Saarbruken, 6, Germany Appl. No. 823,077

Filed May 8, 1969 Patented Oct. 19, 1971 Priority May 8, 1968, May 28, 1968, Aug. 19,

1968, Sept. 3, 1968, Jan. 17, 1969, Sept. 17, 1968 Germany P 17 65 366.6, P 17 65 489.6, P17 65 970.0, P 17 90 058.2, P 19 02 569.5 and P 17 90 142.7

WELDING OF SHEET METAL COATED WITH LAYERS 7 Claims, 41 Drawing Figs.

U.S.Cl 219/91, 219/92 lnt.Cl 823k 9/28, B23k 1 H10 Field of Search 219/91, 92,

[56] References Cited UNITED STATES PATENTS 555,131 2/1896 Thompson 219/94 X 2,441,822 5/1948 Klemperer.... 219/113 X 2,844,705 7/1958 Bowman et al.. 219/123 X 3,068,349 12/1962 Tribe 219/91 3,252,203 5/1966 Alberts et al. 219/91 X 3,515,841 6/1970 King 219/91 FOREIGN PATENTS 697,396 9/1953 Great Britain 219/91 Primary Examiner-J. V. Truhe Assistant ExaminerHugh D. J aeger Attorneys-l eith Misegades and George R. Douglas, Jr.

ABSTRACT: A method for the connection of sheet metal panels coated on at least one surface with an insulating layer by the electrical fusion of a metal, wherein the improvement comprises the steps of supplying an electric current for the fu sion of metal indirectly along the sheet metal of at least one panel to the zone at which said panels are to be connected to each other, and keeping the temperature of an outer insulating layer of at least one of said panels below the melting point of said layer.

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PAIENTEBnm 19 I9" 3, 6 14,37 5 SHEET 12 0F 14 PAIENIEllncI 19 I971 SHEET 1 4 [1F 14 v V a!!! WELDING OF SHEET METAL COATED WITH LAYERS CROSS-REFERENCE TO RELATED APPLICATIONS Johann Karl Wefers and Otto Alfred Becker, Ser. No. 681,694 filed Nov. 9, 1967; Otto Alfred Becker, Ser. No. 704,684 filed Feb. 12, 1968.

BACKGROUND OF THE INVENTION Sheet metal panels coated with synthetic substances are desirable owing to their rust-free, insensitive surfaces available in many patterns and colors. However, such sheet metal panels cannot be connected to each other by spot or roller welding in the usual manner. According to specification Ser. No. 668,414 the sheet metal panels are stripped at the places to be welded in such a manner that an electrical contact is established when the sheet metal panels are pressed upon each other. With comparatively thick layers of synthetic substances and likewise with comparatively thick sheet metal panels the cavities formed by stripping the coating layers may be filled by inserted intermediate welding pieces. In this manner faultless welded connections can be produced with the usual welding machines on sheets of steel, chromium-nickel steel, aluminum or other metals coated with synthetic substances, lacquers or other materials. However, the outer layers of the stripped sheet metal panels are destroyed by stripping or melting under the electrodes.

The present invention has the object of preventing the destruction of at least one outer layer when welding.

BRIEF SUMMARY OF THE INVENTION In a method according to the invention for the resistance welding of sheet metal panels coated preferably with synthetic substances the welding current for at least one of the coated sheet metal panels is conducted indirectly to the welding zone through the sheet metal. Therefrom the advantage results that the outer coating of at least one of the sheet metal panels remains undamaged, and that the welding is performed as an internal welding of the juxtapositioned surfaces of the sheet metal panels.

For welding two sheet metal panels to one another, which are coated on both sides, the upper sheet metal panel may be stripped on top of its coating for placing an electrode on it, while the lower sheet metal panel rests through a cooling plate and an insulator on the lower electrode. This lower electrode is connected by a cable to the lower sheet metal as well as to the upper sheet metal. When switching on the current the same flows firstly from the upper electrode through the upper sheet metal to the lower electrode, whereby the sheet metal is heated and the layer between the sheet metals melts. Under the pressure of the upper electrode the heated upper metal bulges and comes into contact with the lower sheet metal. The current then flows to the lower electrode also through the lower sheet metal, and welds the two sheet metals to one another.

For welding an upper sheet metal panel to two lower sheet metal panels lying side by side, two separate electrodes may be used which are applied to stripped zones of the upper sheet metal panel. The lower sheet metal panels lying on a cooling plate are indirectly connected by cables to an opposite electrode. When switching on the current, firstly a heating current flows horizontally in the upper sheet metal, and after the melting of the coating layers a current flows perpendicularly through the two sheet metals whereby these panels are connected simultaneously on two points to each other.

When using an electrode and opposite annular electrode arranged coaxially to each other for welding, a common stripped zone is provided in the upper sheet metal panel. Moreover, the opposite electrode is connected to the lower sheet metal panel through cables. When switching on the current, firstly a heating current flows horizontally between the electrodes, and after the melting of the coating layers a welding current flows perpendicularly through the two sheet metal panels and welds the same to one another. The annular electrode limits the heating of the upper sheet metal panel to a small area.

For the purpose of roller welding, the upper sheet metal panel is stripped over an elongated zone for the roller electrode, and between the sheet metal panels coated on both surfaces a wire is inserted along the weld seam to be produced.

5 Both sheet metal panels are connected by cables to the opposite pole of the current source. When switching on the current, a heating current flows horizontally through the upper sheet metal until the internal coating layers are melted; then the wire establishes contact between the sheet metal panels and a welding current welds the wire to the sheet metal panels. The wire may be additionally connected to the said opposite pole.

In accordance with a modification, the wire may be interposed between the welding roller and the upper layer of the upper sheet metal panel, and may be connected like the sheet metal panels through regulating resistors to the said opposite pole. This method has the advantage that any stripping is dispensed with.

Alternatively both sheet metal panels may be connected indirectly to the poles of a secondary circuit, placed between cooling plates and pressed against each other by presser means. Between the sheet metal panels a row of wires may be interposed and connected at their ends each to one of said poles. According to a modification two rows of wires crossing one another may be interposed and electrically connected at opposite polarity. When the current is switched on, firstly a heating current flows, and after the melting of the coating layers a welding current. By applying a single pressure means a plurality of welding seams can thus be produced simultaneously. Both outermost coatings thus remain undamaged. Upper and lower vibrator devices may be provided for assisting the welding operation.

On the same principle the angled ends of sheet metal panels may be welded, e.g. those of partition walls. For this purpose the inner welding areas are to be stripped, welding pieces are to be inserted, and each sheet metal is to be connected to a pole of a secondary circuit. When switching on the current and the presser means, the sheet metal panels are welded to each other while the outer coatings remain undamaged.

The sheet metal panels to be welded to one another may moreover be placed between the upper and lower plates of a capacitor, in order to force the welding current flow towards the interior. Thus the warming of the external coatings of the sheet metal panels may be reduced.

According to a further embodiment of the method according to the present invention at least the indirectly connected sheet metal panel may be provided with milled recesses for leaving undamaged another outer coating. In a first example the milled recess leaves a small welding pin in existence, against which the stripped upper sheet metal panel is pressed by a small electrode, and is thus welded to it. In a second example both sheet metal panels are provided with welding pins, connected indirectly to a current source, and welded to each other through an interposed intermediate welding piece. In a third example the milled recess of the indirectly connected sheet metal panel leaves a central point standing, the intermediate welding piece is provided on top with a pin or with an annular edge, and the thickness of the upper sheet metal panel is reduced underneath the electrode. In a fourth example both inner surfaces of the sheet metal are provided with milled recesses of square cross section; in a fifth example with a recess of triangular cross section for connecting a round intermediate welding piece. Thereby the contact areas are reduced practically to contact points or lines only, whereby the re sistance of transition is increased, and the size of the welding zone as well as its warming are reduced.

In accordance with a further embodiment an upper milled 70 recess of the upper sheet metal panel may be provided with a 3 dimple having a downwardly directed point, For this purpose a I single milled recess only is to e provided. Moreover the outer layer lying underneath the welded area may be bought into direct contact with a coolant, apart from the use of cooling 75 plates.

According to a further embodiment intermediate welding pieces of a low melting point material, e.g. of hard or soft solder, are usedsThus the panels need not be heated to the welding temperature of the sheet metal and their warming is reduced.

For the use of capacitor pulse welding, blank metal surfaces may be formed on the internal welding areas, and thus the narrow gaps required for igniting an arc may be established. In the simplest case the two locally stripped internal coatings have together a thickness which corresponds to the gap required. With thicker coatings the gap may be established by impressing a pointed pressure punch. Moreover intermediate weldingpieces having points or ribs projecting downwardly and upwardly may be inserted into the cavity formed by stripping, and the sheet metal panels themselves may be pro vided with sharp-edged ribs where they are being stripped. Through a bore in the upper sheet metal panel a pointed welding pin or a welding pin having an annular edge may be inserted, and pressed into the cavity during welding by a resilient pusher pin. Thus the cavity is filled. The upper sheet metal panel consequently remains plane.

For sheet metal panels coated on one side only, particularly of light metal, cavities are formed by the insertion of an insulating layer having holes for the insertion of very small intermediate welding pieces of high electrical resistance. These small welding pieces may consist of wire squeezed flat between the welding areas, and may be stuck to the perforated insulating strip.

The igniting of the electric arc may be initiated by the aid of welding pins as will be shown with reference to portable manual welding appliances. In accordance with a first embodi ment the upper sheet metal panel is provided with a milled out recess leaving a welding pin standing. The welding appliance capable of being placed on top by means ofa tripod comprises a spring-loaded presser punch, by the impact of which on the sheet metal the electric arc is ignited and the welding is performed.

In accordance with a second embodiment firstly an intermediate welding piece with a welding pin is welded to the lower sheet metal panel by means of a spring-loaded electrode, and subsequently an upper sheet metal panel provided with welding pins is welded to the intermediate welding piece. The sheet metals are thus kept spaced apart.

In accordance with a third embodiment a welding pin having a holder flange is inserted into the cavity formed by the stripping through a bore in the upper sheet metal panel, and is welded on by the aid of a spring-loaded electrode.

In order to displace the flow of current towards the inner welding zone, the external surfaces of the sheet metal panels may be arranged between capacitor plates and the inner sheet metal surfaces may be separated by a capacitor foil of opposite polarity. Thus the warming of the outer coatings is reduced.

According to a further embodiment of the method according to the invention a welding area may be firstly warmed by resistance heating, and subsequently welded by the capacitor pulse method by a changeover switch. The warming of the outing coating is thus reduced and the same is kept undamaged.

Finally the welding appliance may be placed within a vacuum bell and sealed about the zone to be welded in an airtight manner by an interposed rubber ring. The contact pres sure is then not limited to manual force.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS Various embodiments of the method according to the present invention and the appliances required for performing the same will now be described by way of example with reference to the accompanying drawings, in which:

FIG. I is a section of a welded spot of two sheet metal panels each coated on both sides, their outer coatings left undamaged, and having two indirect supplies of welding current and a direct one through an electrode.

FIG. 2 shows two welded spots ofthree sheet metal panels, each coated on both sides, with two indirect current supplies and two direct supplies of current through two separate electrodes.

FIG. 3 shows a welded spot of two sheet metal panels each coated on both sides with indirect current supply and a direct supply of current through an inner and an outer electrode.

FIG. 4 is a transverse section of a prepared weld seam for two sheet metal panels, each coated on both sides and separated by a welding wire, with indirect current supplies and a direct supply ofcurrent through a welding roller.

FIG. 5 is a longitudinal section ofa prepared weld seam for two sheet metal panels each coated on both sides, with s superimposed welding wire,having indirect current supplies and one direct supply of current through a welding roller.

FIG. 6 is a section of a prepared seam welding for two sheet metal panels, each coated on both sides, separated by wires with indirect current supplies, enclosed within a vibrator device.

FIG. 7 is a section of a prepared seam welding for two sheet metal panels, each coated on both sides, separated by crossed wires with indirect current supplies.

FIG. 8 is a perspective illustration of a welding wire having an hourglass-shaped cross section.

FIG. 9 is a perspective illustration of a welding wire having a cusped triangular cross section.

FIG. 10 is a section of a prepared welding spot for a U- shaped folded-over margin and an angled margin engaged therein of two sheet metal panels, respectively, each coated on both sides, with intermediate welding pieces and indirect current supply.

FIG. II is a section of a prepared welding spot of two sheet metal panels, each coated on both sides, with an intermediate welding piece and indirect current supply, enclosed between capacitor plates.

FIG. 12 is a section of a prepared welding spot for two sheet metal panels, each coated on both sides, the lower panel having a milled-out welding pin on its inner surface.

FIG. 13 is a section ofa prepared welding spot for two sheet metal panels, each coated on both sides, with milled-out welding pins and an intermediate welding piece interposed between them.

FIG. 14 is a section ofa prepared welding spot for two sheet metal panels, each coated on both sides, with milled recesses and an intermediate welding piece.

FIG. 15 is a section of a prepared welding spot for two sheet metal panels, each coated on both sides, with milled recesses of rectangular cross sections and a round intermediate welding piece.

FIG. I6 is a section ofa prepared welding spot for two sheet metal panels, each coated on both sides, with milled recesses of triangular cross sections, a round intermediate welding piece and a current supply device.

FIG. 17 is a section of a prepared welding spot for two sheet metal panels, each coated on both sides, the upper panel having an external milled recess and a downwardly directed point, and the lower panel being cooled at the welding zone.

FIG. 18 is a section of a prepared welding spot for two sheet metal panels, each coated on both sides, with a low melting point intermediate welding piece.

FIG. I9 is a modification of FIG. 18 with milled out welding pins.

FIG. 20 is a section of a prepared welding spot for two sheet metal panels, each coated on both sides, with internal stripping for arc welding.

FIG. 21 is a plan view ofa stripped zone with circular ribs.

FIG. 22 shows an intermediate welding disc having ribs on both sides.

FIG. 23 shows an intermediate welding strip having ribs on both sides.

FIG. 24 shows an elongated stripped zone with ribs.

FIG. 25 is a cross section of a prepared welding spot with a downwardly pointed embossed dimple underneath the electrode.

FIG. 26 shows a prepared welding spot with an intermediate welding piece having points directed upwardly and downwardly.

FIG. 27 is a plan view of the welding piece of FIG. 26.

FIG. 28 shows a prepared welding spot with a welding pin inserted.

FIG. 29 shows a modification of FIG. 28 with a welding pin capable of being advanced.

FIG. 30 is a section ofa prepared welding zone for two sheet metal panels, each coated on one side, with an insulating insert layer and intermediate welding pieces.

FIG. 31 is a plan view of an insulating strip with a welding wire stuck on.

FIG. 32 is a section of a prepared welding spot with a welding device superimposed.

FIG. 33 is a section of a prepared welding spot for a lower sheet metal panel with an intermediate welding piece held by the welding device.

FIG. 34 is a similar section with an upper sheet metal panel to be welded to the intermediate welding piece.

FIG. 35 is a section of a prepared welding spot with a welding pin to be welded in.

FIG. 36 is a section of a prepared welding spot for two sheet metal panels, each coated on both sides, with a current displacement device comprising capacitor plates and foil, with the welding device superimposed.

FIG. 37 is a section of a prepared welding spot with a weld ing device superimposed and a prewarming device.

FIG. 38 shows a prepared welding spot with superimposed welding device within a vacuum bell.

FIG. 39 shows a prepared welding zone for welding by the aid of lifter pin ofa pin welding appliance.

FIG. 40 shows the preparation of the welding area of a lower panel with the aid of a sealing foil stuck to it.

FIG. 41 shows the welding of a lower panel with a sealing foil stuck to it to an upper panel, by the aid of a welding pin.

DETAILED DESCRIPTION OF THE INVENTION In FIG. 1 a sheet metal panel is illustrated having an upper coating 11: and a lower coating lb, which panel is to be welded to a second panel having an upper coating and a lower coating 2b. When welding, one of the outer coatings, namely the lower coating 2b of the panel 2 should remain undamaged. For this purpose the panel 1 has a stripped area 1c at the weld ing zone, so that an upper electrode 5 comes into contact with the metal sheet of panel 1. The panels 1 and 2 are connected by cables 7a, 7 to a lower counterelectrode 6. The lower coating 2b rests on a cooling plate 10, and the latter rests through an insulator 12 on the flat counterelectrode 6. When switching on the current, the same flows from the upper electrode 5 through the sheet metal 1 and the cable 7a to the counterelectrode 6. Thereby the panel 1 is warmed underneath the upper electrode 5; the two internal coatings lb, 2a melt; contact is established with the sheet metal 2; current flows through the sheet metal 2 and cable 7 to the counterelectrode and welds the two sheet metal panels to one another under the pressure of the upper electrode. The welding data are so adjusted and the operation is so controlled that the lower decorative coating 2a does not melt. In order that this coating be not affected even during the cooling of the weld spot, it rests on the cooling plate 10, which in turn rests through an insulator 12 on the counter electrode.

FIG. 2 shows a further example, wherein two electrodes are connected in series side by side. Thus an upper panel 1 having coatings 1a, 1b can be connected simultaneously to two lower panels 2,3 having coatings 2a, 2b and 3a, 3b, respectively. The upper panel 1 has stripped zones 1c underneath the right-hand side electrode 5 and underneath the left-hand side electrode 6. The lower panels rest on a cooling plate 10, which rests on a table 11. From the right-hand side electrode 5 a cable 4 leads to the lefthand side panel 2, and from the electrode 6 a cable 7 leads to the right-hand side panel 3, a regulating resistor 8 being connected in each of these cables. When switching on the current, the same flows from the electrode 5 through the sheet metal 1 to the counterelectrode 6. Thereby the panelis heated at the welding zones; the coatings lb, 2a and 1b, 3a, respectively, melt; contact is established to the sheet metals 2 and 3. Part of the current then flows from the right-hand side electrode 5 through the sheet metal panels 1,3 and the cable 7 to the counterelectrode 6, and through the cable 4, the sheet metal panels 2, 1 to the counterelectrode 6, and welds the two sheet metal panels simultaneously on both spots. The coatings 2b, 3b resting on the cooling plate remain undamaged.

In accordance with FIG. 3, welding may be performed by means of an annular counterelectrode. An upper panel I hav' ing coatings 10, lb, has a comparatively large stripped area 1c at the welding zone. The lower panel 2 having coatings 2a, 2b rests on a cooling plate 10 which rests through an insulation 12 on a table 11. The inner electrode 5 is surrounded by an outer annular counterelectrode 6. Both electrodes contact the sheet metal 1 within the stripped area 1c. The inner electrode 5 is connected to one pole ofa secondary circuit S of the welding plant by a cable 4, and the outer electrode 6 is connected by a cable 7a to the opposite pole of this circuit. When switching on the current the same flows from the inner electrode 5 through the sheet metal 1 to the outer counterelectrode 6; the panel 1 is warmed at the welding zone; the coatings 1b, 2a melt; contact is established between the sheet metals 1 and 2; the current then flows from the electrode 5 through the sheet metals 1,2 and the cable 7 back to the current source; and the two sheet metals are welded to one another. The coating 2b resting on the cooling plate remains undamaged.

The method of the present invention is applicable also to roller seam welding. In accordance with FIG. 4 an upper panel 1 having the coatings la, 1b is provided with an elongated stripped area It. A lower panel 2 having the coatings 2a, 2b rests on a cooling plate 10 and an insulator 12. Between the panels or rather their coatings lb, 2a a wire 9 is stretched out where the weld seam is to be formed, in order to facilitate the formation of contact. A welding roller 50 is connected by a cable 4 to one of the secondary circuit 5 of a welding plant, the other pole of which is connected by a cable 7 to the sheet metal 1. When switching on the current the same flows through the cable 4, the welding roller 50 and the sheet metal 1 to the cable 7a; the panel 1 is warmed; the coating 1b melts; the wire 9 establishes contact with the sheet metal 1; the coating 2a melts; the wire establishes contact with the sheet metal 2; the current flows through the sheet metal 2 and the cable 7, and welds the wire to the two sheet metals. The coating 2b resting on the cooling plate remains undamaged.

The wire 9 may be alternatively connected to the cable 4. or its ends may be connected with opposite polarity to the cables 4 and 7, respectively.

Welding may even' be performed without any stripping It. For this purpose an upper panel 1 (FIG. 5) having the coatings 1a, 1b rests directly on a lower panel 2 having the coatings 2a, 2b and resting on a cooling plate 10, which is supported on a table 11 by an insulator 12. A wire 9 is stretched out underneath the welding roller 50 and above the coating 1a. The secondary circuit S of the welding plant has one pole connected by the cable 4 to the welding roller,and the other pole connected by the cable 7 to the sheet metal 2, by the cable 70 to the sheet metal 1, and by a cable 7b to the wire 9. Regulating resistors 8 are connected in the cables 7, 7a, 7b. When switching on the current, the same flows from the welding roller 50 through the wire 9 and cable 7b; the wire 9 is warmed, the layer In melts; the wire 9 establishes contact with the sheet metal 1 and the cable 7 and fuses itself, whereby also the coatings lb, 2a are melted, and welding of the sheet metals 1 and 2 to each other is effected through the cable 7. The coating 2b resting on the cooling plate 10 remains undamaged.

According to a further embodiment of the method of the present invention both outer coatings of the panels may remain undamaged and at the same time a plurality of welding spots may be established simultaneously.

In accordance with FIG. 6 parallel wires 9a are stretched out between an upper panel I having the coatings la, Ib and a lower panel 2 having the coatings 2a, 2b. On top of the upper panel I and underneath the lower panel 2 cooling plates 10, respectively, are arranged. Each of the cooling plates may be followed by a vibrator device 13 comprising an electromagnet and a hand plate or footplate I5 connecting the magnets. For the cooling of the magnet plates 15 contacting the coatings, coolant pipes I9 are provided, which are pressed on the plates [5 by springs 18. The lower vibrator device rests on a table 11. The upper vibrator device can be pressed towards the lower device by the aid of pistons 16 and pneumatic cylinders I7. The secondary circuit S of a welding plant has one pole connected by a cable 7 to the lower sheet metal 2, and the other pole connected by a cable 4 to the upper sheet metal 1. The wires 9a are connected at their respective ends to the poles of the said secondary circuit. For the purpose of welding compressed air is introduced into the cylinders 17, and the alternating current for the magnets 14 is switched on. Vibration of the parts to be welded to each other facilitates penetration of the current through the coatings, and accelerates the welding of the parts by reducing the resistance of transition. When switching on the welding current, the wires 9a are heated, they fuse the adjacent coatings 1b and 2a and establish contact with the sheet metals I and 2, to which they are welded under the pressure of the cylinders 17. The two outer coatings la and 2b remain undamaged.

FIG. 7 shows a similar arrangement. Underneath an upper vibrator device 13 comprising a magnet plate 15 and a cooling plate lies an upper sheet metal panel 1 with coatings la, lb, and a lower panel 2 with coatings 2a, 2b rests on a lower vibrator device. On top of the coating 2a there lie a series of wires 9a, and under the coating 1b a series of wires 9b, which cross the wires 9a. Instead of the second series ofwires a sheet metal 9c covering all the wires 9a may be provided, which on its face contacting the wires may be either blank, or if coated, is stripped on circular or stripeshaped zones. The wires 9a are connected by a cable 4a to one pole, and the wires 9b or the sheet metal 9c by a cable 7a to the other pole of a secondary circuit S. Moreover, the upper sheet metal I is connected to the cable 4, and the sheet metal 2 to the cable 7, i.e. with a polarity opposite to that of the adjacent wires. When switching on the welding current, firstly the wires are fused, starting on the crossing points, then the coatings lb, 2a along the wires, and finally the sheet metals 1,2 come into contact with, and under the pressure of the plate are welded to each other. The outer coatings la, 2b remain undamaged.

Instead of the round cross section wires illustrated in FIGS. 4 to 7, profiled wires having sharp edges may be used.

For example, FIG. 8 shows a wire having an hourglassshaped cross section, and FIG. 9 shows a wire having a cusped triangular cross section, FIGS. 8 and 9 being on a larger scale. Circular or stripe-shaped stripped areas may be provided on the welding spots or lines, and intermediate welding pieces in the shape of wires or lengths of wires may be inserted therein.

FIG. I0 shows an example for angularly turned-up sheet metals eg of partition walls. One margin of a sheet metal panel I with coatings Ia, Ib is folded over under an angle of I80". Before the folding, stripped zones Id, 1d of circular or stripe shape have been provided in the coating 1b at the prospective welding zones. The angularly turned-up margin of a panel 2 having coatings 2a, 2b engages in the bend of panel I. On the turned-up flange stripped zones 2c and 2d, respectively, are provided atthe welding zones.

In the upper stripped zones Id, 2c and in the lower stripped zones 2d, Id welding pieces in the form of discs, strips or wires are inserted, the thickness of which for establishing contact exceeds the sum of the thicknesses of two coatings. The bend of the sheet metal panel I rests through a cooling plate 10 on a table 11, while the upper surface of the panel I is covered by a further cooling plate 10, and can be pressed down by a presser plate 15, piston 16 and pressure cylinder. The panel 1 is connected by a cable 4 to one pole, and the panel 2 by a cable 7 containing a regulating resistor 8 to the other pole of the secondary circuit of a welding plant. When switching on the current, the sheet metals I and 2 are welded to the welding pieces 20. The outer coating In of the panel I remains undamaged.

A further embodiment of the method for maintaining an outer coating undamaged is illustrated in FIG. 11. A panel I with coatings Ia, lb is to be welded to a panel 2 with coatings 2a, 2b. For this purpose thejuxtapositioned coatings lb and 2a have shipped zones 1d, 2c, into which a welding piece in the form ofa wire is inserted. To the outer surfaces of the panels I, 2 an upper capacitor plate 31a and a lower capacitor plate 31b are applied, the upper one being capable of being pressed down through an insulator I2 by a presser plate IS. The upper capacitor plate is connected to one pole of a direct current source, the lower one to the other pole thereof. Moreover the sheet metal I is connected by a cable 4 to one pole, and the sheet metal 2 by a cable 7 to the other pole of a source of welding current, the pole of the upper capacitor plate 310 being of the same polarity as the pole of sheet metal 1, and likewise that of the lower capacitor plate 31b being of that of the sheet metal 2. When the capacitor plates are charged and the welding current is switched on, the electric field of the capacitor plates forces the welding current from the outer layers inward towards the welding point, whereby the welding is accelerated, warming is reduced and kept away from the decorative outer coating. The decorative outer coating remains undamaged even with thin sheet metals and thin coatings.

The capacitor plates may serve at the same time as cooling plates. Their shape may be adapted to a curved row of welding spots or line, and they are capable of being pressed towards each other by pistons and pneumatic cylinders, see FIG. 6. Likewise the vibrator device shown there may be additionally provided.

The method according to the present invention may be carried out also with light metal alloy sheets having oxidized surfaces or with bonderized steel sheets the outer surfaces of which are to remain undamaged. At the welding zones oxide layers, which would require a temperature of 2000 C. for melting, and the bonderized layers are removed by mechanical stripping. Inserts of the same metal or of suitable special alloys are to be inserted into the cavities, which inserts are quickly welded to the blank welding zones while developing little heat.

In order to confine the warming of the panels to a small region, to increase the resistance of transition at the welding Zone and to make do with short welding periods the contact areas on the welding zone are substantially reduced. Accord ing to a first example this can be done simultaneously with the stripping. As exaggeratedly shown in FIG. I2, an upper panel I with coatings Ia, lb is provided on the outer surface with a small stripped zone Id, and a lower panel 2 with coatings 2a, 2b is provided not only with a stripped zone but moreover with a recess 2f milled out of the sheet metal, which reduces the contact area at the welding zone to a very small welding spot 2g. An annular milling cutter cuts out an annular groove 2f and leaves a cylindrical welding pin 2g standing. The lower panel 2 is supported on a table II by a cooling plate 10, while the panel I is depressed by means of a piston I6 and pressure cylinder 17, through an upper cooling plate I0 having a cutout for an electrode 5. The electrode 5 having a small tungsten tip 5b is connected to one pole of the secondary circuit of a weld ing plant, preferably a capacitor welding plant, while the other pole thereof is connected to the sheet metal 2. When switching on the current, the small area of the tungsten tip 5b contacts the sheet metal 1. Under the ressure of the electrode 5 the sheet metal 1 comes into contact with the small welding pin 2g and is welded thereto. The outer coating 2b of the panel 2 remains undamaged, and the outer coating la of the panel 1 is to be recoated on a small area only.

ln accordance with a second example shown in FIG. 13 each of the panels 1 with coatings la, lb and the panel 2 with coatings 2a, 2b is to be provided on the inner surface with a stripped zone If, 2f, respectively, with the formation of a welding pin 1g, 2g, respectively, and a welding piece 20 is inserted between the welding pins, the thickness of which piece exceeds the sum of the thicknesses of the removed coatings. The lower panel 2 rests on a table 11 through a cooling plate 10, while the upper panel 1 is capable of being pressed down through an upper cooling plate by a piston 16 and pressure cylinder 17. The sheet metal 1 is connected by a cable 4 to one pole of the secondary circuit S of a welding plant, and the sheet metal 2 is connected by a cable 7 to the other pole thereof. When switching on the current, the same can flow only through the small contact areas of the welding pins lg,

2g, which are heated quickly and are welded to each other under the pressure of the cylinder 17 while developing little heat. The outer coatings la, 2b thus remain undamaged.

In accordance with a third example illustrated in FIG. 14 the upper panel 1 with coatings la, lb is provided on top with a trough-shaped milled recess lfcorresponding to the shape of the electrode, and on the under side with a stripped zone 1d, and the panel 2 is provided with a conical milled recess 2f leaving a welding point 2g standing. Between the sheet metals an intermediate welding piece 20 is inserted, which has on top a pin 20a and/or an annular edge for reduction of the contact area. The lower panel 2 rests on a table 11 through a cooling plate 10. An electrode connected by a cable 4 to one pole of a secondary circuit S is pressed down into the milled recess 1}" of the upper panel, while the other pole thereof is connected to the sheet metal 2 by a cable 7. When switching on the current, the same can flow through small contact areas only, which are quickly heated and welded to each other. The coating 2b of the lower panel remains thus undamaged.

In accordance with a fourth example illustrated in FIG. 15 a panel 1 with coatings 2a, 2b are each provided on their inner surfaces only with a milled recess of rectangular cross section, 1f, 2f, respectively, and an intermediate welding piece of spherical or wire shape is inserted into the recesses, the diame ter of which intermediate piece exceeds the depth of the two milled recesses put together, and has only a point or line con tact with the bottoms of the milled recesses. The panel 2 rests on a table 11 through a cooling plate 10, while the panel 1 is capable of being pressed down through an upper cooling plate 10 by means ofa piston 16 and pressure cylinder 17. The sheet metal 1 is connected by a cable 4 to one pole of a secondary circuit, and the sheet metal 2 is connected by a cable 7 to the other pole thereof. When switching on the current, the same can flow only through the contact points or lines which are heated quickly and are welded to each other. The outer coatings la and 2b thus remain undamaged.

In accordance with a fifth example illustrated in FIG. 16, a panel 1 having coatings la, lb and a panel 2 having coatings 2a, 2b are provided on their inner surfaces with stripe-shaped stripped zones 1d, 2c, respectively, and within these stripped zones with small milled recesses of triangular cross section If, 2f, respectively; into these milled recesses a welding piece 20 in the form ofa wire is inserted, the diameter ofwhich exceeds the depths of the said recesses put together. The panel 2 rests at its left-hand side end through a cooling plate 10 and an insulator 12 on the lower electrode ofa welding plant, while the upper panel 1 overlaps with its right hand side end the lower panel 2 and is capable of being pressed down through an upper cooling plate 10 and an insulator 12 by the upper electrode 5 of a welding plant. The panels 1, 2 are held in position by upper and lower pairs of pistons 16 and pressure cylinders 17, respectively. The electrodes 5, 6 are connected by a cable 4 and 7, respectively to current supply device 24 contacting the outer cut edges 1s, 25 respectively, of the sheet metals. These devices comprise flexible silver-plated copper strips 240 attached to an elongated plate 24c through resilient insulating strips 24b, e.g. of rubber, and can be clamped by holders 24d, e.g. resilient clips, on the margins of the panels 1, 2 respective ly. The plates 24c are capable of being pressed by pistons 16 and pressure cylinders 17 against the edges of the panels 1,2, respectively. The length of the device depends on the current intensity to be transmitted. When switching on the current, the same flows through the cables 4, 7 into the sheet metals 1, 2 respectively, and through their contact lines with the welding piece, which are quickly heated and welded. The outer coatings la, 2b thus remain undamaged.

According to a modification, the coating 1a, at the dimple 1! and/or the coating 2a of the welding zone underneath may be previously removed.

A further example of welding with little development of heat in the lower decorative panel is illustrated in FIG. 17. An upper holder panel 1 with coatings la, lb has on top a milled recess 1f reducing the thickness of the sheet metal. In the middle of the milled recess a downwardly pointed dimple ll is provided which is produced by the tip of an electrode or by a punch in a separate operation. A lower panel 2 with coatings 2a, 2b rests on a cooling plate 10 having a recess 10c undemeath the welding spot. Coolant supply-and-drain pipes and 10b, respectively, issue into said recess, whereby even a slight warming of the outer coating 2b is prevented which might cause a slightly lustrous spot thereon. An electrode 5 having a bore 5e engages in the milled recess If, in which bore a spring-loaded electrode pin Sm is mounted, having a tip 5n for the dimple ll. The electrode 5 and electrode pin 5m are connected by a cable 4 to the one pole of the secondary circuit S of a welding plant, the other pole of which is connected by a cable 70 through a regulating resistor 8 to the sheet metal 1 and by a cable 7 through a further regulating resistor 8 to the sheet metal 2. When switching on the current, the same flows via the electrode 5 and the sheet metal 1, whereby the latter is warmed at the thinned region at the milled recess; the coating 1b and subsequently the coating 20 melt. The dimple 1! comes into contact with the sheet metal 2; current flows then also through the sheet metal 2 and welds the sheet metal 2 to the sheet metal I under the pressure of the electrode 5. The tip 5n of the spring-loaded electrode pin 5m is thus pushed into the electrode 5 so that a weld blob is formed with little warming of the decorative panel. The outer coating 2b thus remains undamaged.

Further reduction of the heating of the coated panels is possible by making the welding pieces of a material, the melting point of which is below the melting point of the sheet metals to be connected. For steel sheet the usual hard and soft solders, and for aluminum sheet, aluminum alloys are in consideration which have a low melting point.

In accordance with FIG. 18 an upper panel 1 1 with coatings la, lb is provided with a stripped zone lc on top and a stripped zone 1d on its underside, and a lower panel 2 with coatings 2a, 2b is provided with a stripped zone 2c on top, these stripped zones having the shape or circles or strips. In the cavity formed by the inner stripped zones there is inserted an intermediate piece 21 in the form of a disc or strip and of a material the melting point of which is lower than that of the sheet metals 1,2. The lower panel 2 rests through a cooling plate 10, e.g. of copper, on an electrode 6. An upper electrode 5 engages in the stripped zone 1c of the upper panel. The lower sheet metal is connected by a cable 7 to the lower electrode 6. When switching on the current, the intermediate piece 21 melts and connects itself to the sheet metals, whereafter the current is switched off before the melting point of the sheet metals is reached. The outer coating 2b of the lower panel thus remains undamaged.

In accordance with FIG. 19 the warming of the sheet metals is reduced and the resistance of transition at the contact areas is increased. For this purpose the panel 1 with coatings la, lb and the panel 2 with coatings 2a, 2b are provided at their inner surfaces will milled recesses, 11', 2f, respectively, leaving small welding pins lg, 2g, respectively, standing in their centers. The milled recesses are annular for spot welding, and in the 

1. A method for the connection of sheet metal panels coated on at least one surface with an insulating layer by the electrical fusion of a metal, said sheet metal panels having peripheral edges being uncoated throughout a portion of its edge surface, at least one of said insulating layers being an outer insulating layer, wherein the improvement comprises the steps of removing a portion of said outer insulating layer forming an exposed zone, supplying an electric current through a welding electrode placed essentially adjacent an area of said portion of said outeR insulating layer for the fusion of metal essentially parallel to the surface of the sheet metal from a point on said edge surface and providing a return path through said electrode, and keeping the temperature of said outer insulating layer of at least one of said panels below the melting point of said layer.
 2. A method as claimed in claim 1, comprising the step of subjecting the zones, at which said panels are to be connected to each other, to vibration.
 3. A method as claimed in claim 1, comprising the step of cooling the coating of the surface of at least one of said panels which does not contact another of said panels.
 4. A method as claimed in claim 1, comprising the step of melting the metal by electrical resistance welding.
 5. A method as claimed in claim 1 wherein an insulating layer is interposed between the contacting surfaces of the metal sheets and comprising the steps of stripping zones at opposite surfaces of sheet metal panels of said insulating coating, temporarily superficially melting said zones by electric arc discharges, and pressing said superficially melted zones against each other for metallic connection thereof.
 6. A method as claimed in claim 1, comprising the step of melting the metal by the current of a capacitor pulse discharge.
 7. A method according to claim 1, comprising the step of reducing the contact zone and thereby increasing the transfer resistance of the electrical current from one of said panels (1, 2) to the other. 